Field of the Invention
The present invention relates to a capsule endoscope, and more particularly, to disposition of illumination control circuits installed in a capsule endoscope and a wiring between the circuits.
Description of Related Art
In the related art, to examine the inside of a body, a capsule endoscope has been used in which an endoscope system provided by an illumination unit, an optical system, an imaging element, a signal-processing unit, a communication antenna, a power supply, and so on, is provided in a tubular capsule housing having hemispherical end portions.
In such a capsule endoscope, components that configure the endoscope system are mounted at positions separated in a flexible substrate formed as one substrate. In such a capsule endoscope, a wiring region in the flexible substrate is formed such that wirings configured to perform transmission and reception of signals between the components are formed in the wiring region, and the wiring region in the flexible substrate is accommodated in the capsule housing in a state of being folded to form a mountain fold or a valley fold. The flexible substrate usually has three component mounting region provided by an illumination substrate section on which the illumination unit is mounted, an imaging element substrate section on which the imaging element is mounted and a signal-processing substrate section on which the signal-processing unit is mounted, and two wiring regions disposed between the component mounting regions. More specifically, the flexible substrate is integrally formed in a shape in which the substrate sections are arranged in a row in sequence of the illumination substrate section, the wiring region (hereinafter, referred to as “a first wiring substrate section”) between the illumination substrate section and the imaging element substrate section, the imaging element substrate section, the wiring region (hereinafter, referred to as “a second wiring substrate section”) between the imaging element substrate section and the signal-processing substrate section, and the signal-processing substrate section.
The illumination unit provided in the capsule endoscope is provided by a plurality of white LEDs (light-emitting diodes) serving as the light-emitting element to radiate light to a subject to be photographed, which are connected to each other in parallel, and the white LEDs are disposed at predetermined intervals in the illumination substrate section. An LED-driving circuit provided in the signal-processing unit controls brightness of light emitted by the white LEDs by controlling a voltage of the white LEDs. When a voltage is directly applied to the white LEDs, since the white LEDs are broken by overcurrent, resistive elements configured to prevent breakage due to the overcurrent are serially connected to the white LEDs. That is, the illumination unit has a configuration in which a plurality of sets, each in which the resistive elements and the white LEDs are serially connected (hereinafter, referred to as “a white LED group”), are connected to each other in parallel.
In LEDs, there is a deviation in forward drop voltage. When the signal-processing unit applies a predetermined constant voltage to each of the white LED groups, a deviation occurs in the voltage applied to the resistive elements in the white LED groups due to the influence of the deviation of the forward drop voltages of the white LEDs. Accordingly, the current flowing through the resistive elements and the white LEDs deviates at each of the white LED groups, and a deviation also occurs even in the brightness of the light emitted from the white LED. In the capsule endoscope, it is not a suitable state since the light (the illumination light) cannot be uniformly irradiated to the photographed subject, when the brightness of the white LEDs provided in the illumination unit has variance in light emission luminance.
Therefore, a technology to control light emission of an LED using current is considered, as disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-319707. In the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-319707, an LED-driving circuit provided by a constant current-generating circuit and a current mirror circuit corresponding to LEDs to cause the same current to flow to the LEDs is disclosed. Since the configuration of the LED-driving circuit disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-319707 is applied to the signal-processing unit as the LED-driving circuit configured to control the illumination unit provided in the capsule endoscope, the plurality of white LEDs that configure the illumination unit can be driven at a low voltage in a state in which a deviation in brightness is suppressed. In such a capsule endoscope, it is possible to increase a number of the white LEDs to improve the brightness (luminance) and the uniformity of the illumination light irradiated onto the subject. In the disclosed technology, light emission of the LEDs is controlled by using the current by one ground signal commonly connected to cathode terminals of all of the LEDs and signals of current mirror circuits connected to anode terminals of the LEDs, respectively, i.e., signal lines corresponding to the number of LEDs+1. In addition, as described above, in the capsule endoscope, control of the illumination unit is performed by the LED-driving circuit provided in the signal-processing unit. Accordingly, the signals configured to control the illumination unit that configures the capsule endoscope are input into the illumination unit mounted on the illumination substrate section from the signal-processing unit mounted on the signal-processing substrate section through the second wiring substrate section, the imaging element substrate section and the first wiring substrate section.
When the technology of the LED-driving circuit disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-319707 is applied to the capsule endoscope, if the number of the white LEDs is increased to achieve improvement in brightness (luminance) of the light irradiated onto the subject or uniformity of the illumination light, a larger amount of signals for the illumination unit pass through the second wiring substrate section, the imaging element substrate section and the first wiring substrate section. That is, in the capsule endoscope to which the technology of the LED-driving circuit disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-319707 is applied, the signal lines corresponding to the number of the white LEDs+1 provided in the illumination unit passes through the second wiring substrate section, the imaging element substrate section and the first wiring substrate section.
The case in which the flexible substrate in which components that configure the capsule endoscope are mounted is provided in the capsule housing is considered. As described above, in the capsule endoscope, the wiring region of the flexible substrate, i.e., a portion of the first wiring substrate section and a portion of the second wiring substrate section are folded to be provided in the capsule housing. When the large number of signal lines pass through the first wiring substrate section and the second wiring substrate section by increasing the number of the white LEDs to achieve improvement in luminance (brightness) of the light irradiated onto the subject and uniformity of the illumination light, widths of the first wiring substrate section and the second wiring substrate section are increased according to the number of signals, and a region through which the wirings of the signals pass should be secured.